Pulse-coded communication is employed in applications involving communications over short distances at high bit rates. This applies in particular to wireless personal area networks (WPAN), networks of sensors, radio-frequency identification (RFID) systems, intrusion detection applications, etc.
The use of pulse-coded communication will grow if high bit rates can be achieved at low cost.
In the field of pulse-coded communication, the information to be sent is coded in the form of symbols.
The signal carrying these symbols is not a continuous signal but takes the form of pulses of very short duration.
Each symbol is allocated a transmission time known as the symbol time. The symbol transmission time is divided into time intervals known as frames individually carrying a pulse. Each frame is itself divided into time intervals known as slots. A symbol is thus sent in the form of a set of pulses sent individually in a particular time slot of a frame of the symbol time.
An ultra-wideband signal has certain characteristics.
The frequency band of such a signal is wide (at least 500 megahertz (MHz)). The mean power spectral density per hertz (Hz) of a UWB signal is low. For example, the American Federal Communications Commission (FCC) authorizes a mean power spectral density of the order of −41.3 decibels referenced to one milliwatt per hertz (dBm/Hz) for a signal at a frequency in the range 3.1 gigahertz (GHz) to 10.6 GHz.
There exist several methods of generating and sending a high-bit-rate pulse-coded ultra-wideband signal.
These methods include on/off modulation, which consists in showing a symbol by the absence or the presence of a pulse, and phase modulation, which consists in showing a symbol by a pulse or its complement. The modulation may also be M-ary Pulse Position Modulation (M-PPM), which consists in applying a time shift at the time of sending the pulses as a function of the value of the symbol.
Pulse-coded communication makes it possible to obtain relatively high bit rates (of the order of 480 megabits per second (Mb/s)) but requires a complex and therefore costly architecture, especially in receivers.
The complexity and cost of the architecture may be reduced by using an energy-detecting ultra-wideband receiver, but such a receiver is not suitable for high bit rates. For a high bit rate, in addition to the energy of the noise, it is necessary to take account of the energy of numerous forms of intersymbol interference caused by the transmission channel. The degraded performance caused by such interference may be attenuated if the UWB receiver includes an equalizer, especially a probabilistic equalizer. A probabilistic equalizer takes account of the distribution of the received energy at the level of the time slots of a symbol time.
A probabilistic equalizer of this kind is described by S. Mekki et al. in the document entitled “Probabilistic equalizer for ultra-wideband energy detection”, IEEE 67th Vehicular Technology Conference (VTC), pp. 1108-1112, May 2008. That equalizer considers all possible combinations of interference that might exist for a finite number of symbols. That solution requires the parameters of the transmission channel to be available at the receiver in order for the receiver to effect the equalization.
In the document entitled “EM channel estimation in a low-cost uwb receiver based on energy detection”, IEEE International Symposium on Wireless Communications systems 2008 (ISWCS 08), 2008, S. Mekki et al. propose a solution for estimating transmission channel parameters in the receiver from received information. That solution is based on using the expectation maximization (EM) algorithm as applied to the field of energy. The EM algorithm has the drawback of being complex at the combinatorial level. The iterations required to estimate the channel parameters generally introduce a latency time that compromises the efficacy of the system.